Cometary Big Dig

In the movie "Deep Impact," NASA astronauts try to prevent a comet from hitting Earth. When the mission fails, as a last desperate act they fly their spacecraft into the comet, blowing it to smithereens.

Although there are no comets currently threatening Earth, NASA’s Deep Impact mission also will have a spacecraft fly directly into a comet. Rather than blow up the comet Tempel 1, scientists hope the impact will excavate a crater on the surface, allowing them to see the interior of a comet for the first time.

The comet Tempel 1 formed in the Kuiper Belt, a gas and dust-filled region in the solar system located beyond Neptune’s orbit. Tempel 1 now orbits the sun every 5.5 years, tracing a path between the orbits of Mars and Jupiter.

The impact will alter the course of the comet’s orbit, but mission scientists say this won’t push the comet on a path toward Earth.

"The impact will make a change in the orbit of the comet, but the change will be so small as to be undetectable," says University of Maryland astronomy professor Michael A’Hearn, Deep Impact’s principal investigator. "To put it in perspective, the entire comet nucleus would easily fit inside the city of Washington. The distance of the comet from the Earth at its closest approach to the Earth’s orbit is half an astronomical unit, or 50 million miles. We expect that we cannot change the orbit of the comet any more than moving the city of Washington’s center from the Capitol to the end of the Mall."

The rocks inside a crater on the Asteroid Eros, as imaged before impact with the NEAR spacecraft. Numerous small impacts on the asteroid show brown boulders visible interior to the less exposed (white) lip of the crater. False-color for emphasis.Image Credit: NASA/Eros

When the Deep Impact spacecraft approaches Tempel 1 on July 4, 2005, it will release a smaller spacecraft designed to impact the comet. This impactor spacecraft is equipped with a camera that will take pictures as it approaches the comet’s surface. The primary part of the spacecraft that will hit the comet is a copper disc weighing over 300 pounds.

"Copper is a part of the same part of the periodic table as gold and silver, but it’s cheaper," says A’Hearn, "All those metals – copper, gold, silver – don’t react with water very efficiently; they’re the noble metals. We want our impactor not to react with the cometary materials. If we made the impactor entirely out of aluminum, the aluminum would react very rapidly with the water, and we would get bright emissions from aluminum oxide, making it very difficult to see the things that we’re interested in, in the spectra."

The impact spacecraft will fly head-on into the comet at 23,000 mph. According to A’Hearn, some scientists think the impact could fracture the comet into several pieces, while others believe the impact will merely compress the cometary materials. But he says the most-hoped for and expected scenario is that the spacecraft will blow open a crater ranging somewhere between two to fourteen stories deep on the surface of the 6-kilometer wide comet, spraying dust, gas and ice into space for hours afterward.

By looking at the spectra of the ejected impact material, scientists will be able to determine the chemical composition of that material. Comets are often called dirty snowballs because they are made out of water ice and organic materials. Most of the comets in the solar system formed 4.5 billion years ago, at the same time the planets were forming. Scientists believe that comets, with their unaltered chemistry, can tell us what the solar system was like in its earliest days.

Comet Halley imaged by European flyby. Credit: ESA

By crashing a spacecraft into a comet, scientists hope to get their first look at what kind of materials lie just beneath the comet’s surface. As a comet travels near the sun, the surface of a comet changes as it becomes heated, but presumably the material under the surface is more pristine. That information could help scientists answer a few unanswered questions about the nature of comets.

"Comets eventually stop outgassing – why is this?" asks A’Hearn. "Is it because they’ve exhausted all the gases in the interior – all the ices have evaporated and the gas has flown out – or is it because the surface layer has developed a crust, an organic goo or just a weight of overbearing materials that prevents the ice inside from evaporating and escaping as gas?"

The flyby spacecraft will take photos of the comet before, during and after the impact spacecraft crashes into the comet, and then will transmit that data to Earth over 30 days.

Ground-based telescopes around the world will be trained on the comet at the time of impact, as well as all the observatories atop Mauna Kea in Hawaii. The Hubble, Chandra and Spitzer Space Telescopes also will be used to observe the impact.

Amateur astronomers might be able to observe the impact as well. Although comet Tempel 1 normally is not bright enough to observe with the naked eye, the impact could cause the comet to brighten considerably. Sky watchers should look for the comet near the bright star Spica and the planet Jupiter early in the morning on July 4.

The painting titled "K/T Hit" by artist Donald E. Davis. This impact occured 65 million years ago, ending the reign of the dinosaurs.Image Credit: Don Davis

The comet nucleus is not clearly visible from Earth, because as the comet approaches the sun, solar heating causes dust and gas to fly off the nucleus, forming the comet’s tail. All that material surrounding the nucleus prevents a clear view.

Scientists got their first close-up look of the nucleus of a comet with the Stardust mission. The comet Wild 2 turned out to be more complex than anticipated, with multiple spires, craters, and jets of gas. Comet Tempel 1 is about the same size as Wild 2, so scientists are curious to see if it is similar in appearance.

The dust surrounding Tempel 1 could create problems for the Deep Impact spacecraft. Both the flyby ship and the impact ship could be sand-blasted by dust streaming of the comet. This may damage the camera attached to the impacting spacecraft, preventing it from taking photos in the last minute or so before the crash.

The Deep Impact mission is scheduled for launch no sooner than January 12, 2005. If spacecraft does not launch by January 28, mission scientists will have to choose another comet target.